1. Field of the Invention
The invention relates to a running control apparatus and a running control method for a vehicle such as an automobile or the like. More specifically, the invention relates to a running control apparatus and a running control method for a vehicle that control a driving force of the vehicle or a driving torque of a driving system (an engine, a motor, or the like) such that a vehicle speed of the vehicle coincides with a target vehicle speed.
2. Description of the Related Art
In recent years, vehicles such as automobiles and the like have been mounted with “automatic running control apparatuses” for automatically controlling vehicle speed through, for example, “cruise control”, “creep running control” (Japanese Patent Application Publication No. 2004-90679 (JP-A-2004-90679)), “downhill assist control” (Japanese Patent Application Publication No. 2006-213294 (JP-A-2006-213294)), and the like. When such an automatic running control apparatus is actuated, a target value of a vehicle speed (target vehicle speed) is first decided according to an arbitrary method, for example, through designation or selection of the vehicle speed by a driver or on the basis of a running state of a vehicle (or a maneuvering state of the driver) or the like. Then, a difference between the target vehicle speed and an actual value of the vehicle speed (actual vehicle speed) is transmitted as feedback to a control device (driving control device) for a driving system of the vehicle such as an engine, a motor, or the like, and a driving force or a driving torque is controlled such that the actual vehicle speed coincides with the target vehicle speed, namely, that the difference between the target vehicle speed and the actual vehicle speed is eliminated. According to automatic running control as described above, the driver is not required to adjust a depression amount of an accelerator pedal or a brake pedal for controlling the vehicle speed to a certain value or holding the vehicle speed constant. Therefore, the burden imposed on the driver in maneuvering the vehicle is drastically reduced. The aforementioned automatic running control apparatus can also be used in, for example, remotely operating the running of the vehicle.
In feedback control based on the difference between the target vehicle speed and the actual vehicle speed in the aforementioned automatic running control apparatus, a required driving force or a required driving torque is often decided using an integrated value (or an accumulated value) (over time) of the difference between the target vehicle speed and the actual vehicle speed to ensure good follow-up performance of the actual vehicle speed to the target vehicle speed. A driving force or a driving torque needed to achieve or maintain a certain vehicle speed varies depending on the running state of the vehicle. Therefore, in adjusting the driving force or the driving torque using a quantity proportional to the difference between the target vehicle speed and the actual vehicle speed, it is difficult to appropriately set a proportional coefficient thereof, namely, a feedback gain in feedback of the difference between the target vehicle speed and the actual vehicle speed (the responsiveness of the actual vehicle speed deteriorates when the gain is small, and the actual vehicle speed tends to overshoot when the gain is large). On the other hand, in adjusting the value of the driving force or the driving torque on the basis of the integrated value of the difference between the target vehicle speed and the actual vehicle speed, the adjustment amount of the driving force or the driving torque can be increased as the difference between the target vehicle speed and the actual vehicle speed increases, in comparison with the case of making the adjustment using the quantity proportional to the difference between the target vehicle speed and the actual vehicle speed. Even when the feedback gain is held small to prevent the occurrence of overshoot, the actual vehicle speed can be made to swiftly follow the target vehicle speed. The vehicle speed as a target value of control is a first-order lag of the driving force or the driving torque that is changed in control. Therefore, a delay in the response of the vehicle speed for a change in the driving force or the driving torque is compensated for by controlling the driving force or the driving torque on the basis of the integrated value of the difference between the target vehicle speed and the actual vehicle speed.
Incidentally, in an actual vehicle, various types of control (interrupt control) for controlling vehicle speed, driving force, or driving torque are performed in some cases during the actuation of an automatic running control apparatus as described above, interrupting the actuation or in preference to the actuation. For example, when the rotational speed of an engine becomes excessively high with respect to the rotational speeds of wheels and an automatic transmission is overheated while a driving torque is transmitted from the engine to each of the wheels via the automatic transmission, the automatic transmission sometimes performs in some cases control for reducing the rotational speed of the engine (stall control) or the like for a control device for the engine. In such cases, the adjustment of the driving torque for vehicle speed control by the automatic running control apparatus is not achieved. As a result, the actual vehicle speed deviates from the target vehicle speed.
In the control configuration of the aforementioned automatic running control apparatus, if the actual vehicle speed remains deviant from the target vehicle speed (of the automatic running control apparatus) during the performance of interrupt control, which is performed in preference to the control of the automatic running control apparatus, the integrated value of the difference between the target vehicle speed and the actual vehicle speed increases. In consequence, the adjustment amount of the driving force or the driving torque required of the driving system through feedback control by the automatic running control apparatus (the required driving force or the required driving torque) keeps increasing as well. In this situation, when vehicle speed control is resumed in response to the termination of interrupt control and the reestablishment of a state where the control by the automatic running control apparatus is effectively reflected, the driving system such as the engine or the like is actuated with the increased adjustment amount of the required driving force or the required driving torque. Thus, a sudden rise or fall in an actually generated driving force or an actually generated driving torque (the actual driving force or the actual driving torque) is caused. In some cases, as a result, the behavior of the vehicle is seriously disturbed, or a sudden rise or a stagnation in vehicle speed is caused. As a matter of course, it is also conceivable to stop the actuation of the automatic running control apparatus during the performance of interrupt control. In such a case, however, the response of the actual vehicle speed in returning to the target vehicle speed may be delayed when vehicle speed control by the automatic running control apparatus is resumed.
In the related art, however, as described above, there seems to be no proposal to control a vehicle speed in consideration of a case where interrupt control has been performed during the actuation of an automatic running control apparatus.